The present invention relates to an electrophotographic image forming apparatus, and more particularly relates to an image forming apparatus which forms an electrostatic latent image on a high .gamma. photoreceptor by a modulated light beam which has been modulated according to digital image data sent from a computer.
Recently, in the field of electrophotography in which an electrostatic latent image is formed on a photoreceptor and the latent image is developed so that a visual image can be obtained, a digital system of image forming has been actively investigated by which improvements, conversion and editing of images are easily conducted so that image forming of high quality is possible.
An optical scanning system which directly modulates the laser intensity utilizing a semiconductor laser, is used for modulation conducted according to the signal sent from a computer or a document in this image forming apparatus. A dot-shaped exposure is conducted on a photoreceptor, which has been charged uniformly beforehand, by the aforementioned optical scanning system so that a dot-shaped image can be formed.
The section of a beam which is illuminated by the aforementioned optical scanning system, is circular or oval, and its luminance distribution is similar to a normal distribution, both ends of which are spread to the right and left. For example, in the case of a semiconductor laser beam, the luminance is usually 1-6 mW, and its section on a photoreceptor is a very narrow circle or oval of which one or both of the primary and subsidiary scanning lengths is 20-100 .mu.m.
FIG. 10 is a schematic illustration showing the outline of the characteristic of a low .gamma. type photoreceptor.
A low .gamma. type of photoreceptor in which light decay is sharp in the beginning of exposure and gentle in the middle of exposure as shown in FIG. 2a, has been used as a photoreceptor which is applied to an electrophotographic image forming apparatus.
Concerning the low .gamma. type of photoreceptor, the following have been widely known: a mono-layer type such as Se, CdS and the like; and a two-layer type composed of an electric charge generating layer and an electric charge conveyance layer, the two-layer type being usually used in OPC. Light sensitivity of many of the photoreceptors which show the aforementioned semiconductor characteristic, is generally low in a low electric field, compared with a high electric field, and when electric potential is lowered due to an increase in the amount of light, the sensitivity is lowered. For that reason, the information concerning the surface potential in the low luminance region is important. Specifically, the surface potential on the low .gamma. photoreceptor which has been formed as an electrostatic latent image, is subsequently detected, and exposure is conducted according to the detected surface potential in order to control the charging potential. In the way described above, the influence can be prevented which is caused by the fluctuation of photosensitivity due to the change of environmental factors. Further, the charging potential is controlled in order to compensate the fluctuation of photosensitivity caused by the deterioration of the photoreceptor.
If an electrostatic latent image were formed on a low .gamma. photoreceptor by beams emitted from the aforementioned optical scanning system, then a sharp dot-shaped latent image could not be formed because the sensitivity of the aforementioned photoreceptor is generally high in the beginning of exposure, so that the fluctuation of the photoreceptor tends to be picked up.
In this case, there are problems as follows. Even when an electrostatic latent image formed by the aforementioned beams is preferably developed by the method of reversal development, the sharpness of the obtained image is low in many cases. Further, recording of high density is difficult.
In order to take measures to meet the situation described above, the inventors have developed an image forming apparatus having a high .gamma. type photoreceptor, the light decay characteristic of which is as follows: the light decay of the charging potential is not sensitive to a small amount of light so that the charging potential is not decayed when the photoreceptor receives a small amount of light; and when the photoreceptor receives a medium amount of light, the charging potential is sharply decayed.
With a view to solving the conventional problems described above, the first object of the present invention is to provide an image forming method characterized in that: a sharp latent image is formed without being influenced by the change of sensitivity of a photoreceptor caused by the fluctuation of environmental factors; and further the middle tone of the image can be accurately reproduced by dot exposure.
By the image forming method to attain the first object of the present invention, a latent image is formed on a high .gamma. photoreceptor when a modulated beam sent from an optical scanning system is illuminated on the photoreceptor and reversal-development is conducted, and the aforementioned image forming method is characterized in that: the maximum amount of light I.sub.0 in the light amount distribution on the aforementioned photoreceptor, and the amount P.sub.1/2 of light of the half decay exposure satisfy the following inequality; EQU 1.2.times.P.sub.1/2 .ltoreq.I.sub.0 .ltoreq.2.5.times.P.sub.1/2
Further, the present invention is characterized in that: the aforementioned modulated beam is pulse-width-modulated.
The second object of the present invention is to provide an image forming apparatus in which an image of stable quality can be formed without being influenced by the change of light sensitivity of a high .gamma. type photoreceptor, wherein the change of light sensitivity is caused by the fluctuation of environmental factors.
The second object of the present invention can be attained by an image forming apparatus in which a high .gamma. type photoreceptor is illuminated with a modulated beam sent from an optical scanning system so that a latent image is formed and reversal-development is conducted, and which comprises: a half decay exposure amount detection means which detects the half decay exposure amount P1/2 by which surface potential V.sub.0 of the aforementioned photoreceptor can be reduced to 1/2; and a light emission amount setting means which sets the amount of light emitted by a semiconductor laser to a predetermined value according to the result of detection conducted by the aforementioned half decay exposure amount detection means.
The aforementioned light emission amount setting means is characterized in that: the amount of light emitted by the semiconductor laser is set to 1.2-2.5 times that of the half decay exposure amount P.sub.1/2.